1. Field of the Invention
The present invention relates to a method for production of nonreducible dielectric ceramic composition and, more particularly, a method for production of a nonreducible ceramic composition used as a dielectric material for monolithic ceramic capacitors for example.
2. Description of the Prior Art
In general, monolithic ceramic capacitor are produced by a process comprising the steps of preparing ceramic green sheets, forming an internal electrode layer on each green sheet, stacking the green sheets to form a monolithic multilayer body, cutting it into green capacitor chips, firing them in a suitable atmosphere and then forming external electrodes on opposite sides of the resultant capacitor chips.
As a dielectric material for such monolithic ceramic capacitors, there have been used dielectric ceramic compositions of barium titanate modified with zirconates and titanates of alkaline earth metals. Since these dielectric ceramic materials of the prior art are generally reduced to semiconductors when fired in a neutral or reducing atmosphere with a low-oxygen partial pressure, they must be fired in an oxidizing atmosphere.
These conditions require use of an internal electrode material which does not melt at sintering temperature of the dielectric ceramic material and which does not oxidize even in an atmosphere with a high-oxygen partial pressure. For this reason, noble metals such as palladium or platinum have been used as a material for internal electrodes. However, the use of noble metals has resulted in the increase in the cost of monolithic ceramic capacitors as the noble metals are expensive. In addition, there is an increasing demand for monolithic ceramic capacitors which are small in size but large in capacitance. Thus, the cost occupied by internal electrodes increases with decrease in size of monolithic ceramic capacitor.
To solve these problems, it has been proposed to use inexpensive base metals such as nickel as a material for internal electrodes. However, these base metals are easily oxidized in the sintering atmosphere with a high partial pressure of oxygen. Thus, in order to prevent internal electrodes from oxidation during sintering, it is required to use a nonreducible dielectric ceramic material which is not semiconductorized even if fired in a neutral or reducing atmosphere with a low partial pressure of oxygen and which possesses high insulating resistance and excellent dielectric properties enough to use it as a dielectric material for capacitors.
JP-B H2-63664 discloses a nonreducible dielectric ceramic composition composed of a solid solution of barium titanate containing a rare earth element such as Ce. This composition is very useful as a dielectric material for monolithic ceramic capacitors because of its high dielectric constant and high resistance to reduction. Such a dielectric ceramic material has generally been produced by mixing carbonates and/or oxides of the respective components in given molar ratios and then calcining the mixture in air. It is, however, difficult with such a process to prepare raw materials having grain size of not more than 1 .mu.m. If raw materials with large grains size are mixed and calcined to prepare a dielectric ceramic composition, the concentration of the rare earth element varies with place to place because of its nonuniform diffusion, thus making it difficult to produce a dielectric ceramic material with a uniform composition. If such a dielectric ceramic material is fired in a reducing atmosphere, it is partially semiconductorized in the regions where the concentration of the rare earth element is increased. For these reasons, the use of such a dielectric ceramic material results in decrease in the reliability of the monolithic capacitors.